The pathogenesis of human (h) asthma and allergy is marked by dysregulated expression of the Th2 cytokines IL4, which is critical for Th2 differentiation and IgE synthesis, and IL13, which is the central effector of allergic inflammation in the lung. The overall goal of this application is to characterize and dissect the mechanisms by which natural polymorphisms in human IL13, a gene strongly associated with asthma and allergy in human populations, affect Th2 cytokine expression and function, thereby influencing disease susceptibility. To this end, we will rely on a novel, powerful mouse model we recently developed under the aegis of R21A1076715 (Vercelli). Our mice carry a 160 kb BAC transgene that encompasses hRAD50, hIL13 and hIL4, and directs faithfully regulated expression of hTh2 cytokine genes. Preliminary analyses have demonstrated striking increases of human Th2 cytokine production, particularly IL4, in mice that carry nine linked allergy/asthma associated polymorphisms in the human IL13 locus. It is noteworthy that these abnormalities recapitulate alterations we have found in children who carry the same IL13 haplotype. The current application draws its rationale from our previous work, but extends its significance and scope. We now propose to identify the human IL13 polymorphisms responsible for dysregulated hIL13 and hIL4 expression in BAC5 TG mice, and to characterize hIL13 haplotype-specific regulatory and effector events in the lungs of mice further humanized by reconstitution of hIL-4 responsiveness and elimination of endogenous Th2 cytokines. This novel feature of our model will critically add to its ability to assess how hIL13 locus polymorphisms affect Th2-regulated in vivo processes, particularly lung physiology. Our specific objectives are: Specific Aim 1: To identify the hIL13 polymorphisms responsible for dysregulated hIL13 and hIL4 expression in CD4 T cells and mast cells from Hap I-V BAC5 TG mice Specific Aim 2: To generate hIL13 Hap I-V/hIL4R TG/mIL13-/-, mIL4-/- mice that exhibit hIL13 haplotypespecific patterns of hTh2 cytokine expression and respond to hIL-4 and hIL-13 in the absence of endogenous Th2 cytokines Specific Aim 3: To characterize hIL13 haplotype-dependent patterns of lung pathology in hIL13 Hap I-V/hIL4R TG/mIL13-/-, mIL4-/- mice, and identify the regulatory and effector pathways selectively dysregulated by hIL13 polymorphisms We expect these analyses will define the role played by hIL13 variants in Th2 cytokine dysregulation and Th2-mediated lung pathology, thereby revealing fundamental mechanisms of genetically determined susceptibility to allergic asthma.